To me it's the album Rum, Sodomy, & the Lash. It was produced by Elvis Costello and his goal was to capture the Pogues in their live glory, which I think he succeeded at doing in spades. That's really classic Pogues at the peak of their powers. If I Should Fall from Grace with God is also a great album and is the sound of them cresting the wave. After that I'm afraid it was all downhill, mostly because of a lot of problems involving substance abuse and alcoholism.